41 16
Journal of the American Chemical Society
Four cycles of least-squares refinement yielded convergence with final agreement indexes R, = 0.049 for all data and R = 0.069 and R, = 0.049 for observed data. The final "goodness of fit" was 1.32. A final difference Fourier contained three peaks of intensity between 0.51 and 0.73 e/A3 which were associated with the iron atom. No other peaks of intensity greater than 0.50 e/A3 were observed. The final atomic positional parameters and anisotropic temperature factors arc listed in Table VI. Cumulant expansion coefficients for the carbonyl atoms are listed in Table VII. Lists of observed and final calculated structure factor amplitudes and the root mean square amplitudes of thermal vibration are available.23
Acknowledgment. This research has been supported by the National Science Foundation under Grants N S F DMR-7601058, MPS-74-2300, and MPS-76-11833 and is gratefully acknowledged by J.J.W., C.F.P., G.D.S., and E.L.M. Supplementary Material Available: A listing of root-mean-square amplitudes of vibrations and structure factor tables (14 pages). Ordering information is given on any current masthead page.
References and Notes (1) (a) Part 12 in this series: E. L. Muetterties, W. R. Pretzer, M. G. Thomas, B. F. Beier, D. L. Thorn, V. W. Day, and A. B. Anderson, J. Am. Chem. SOC., in press. (b) University of Illinois at Urbana-Champaign. (c) Cornell University. (2) F. A. Cotton, Chem. SOC.Rev., 4, 27 (1975). (3) F. A. Cotton, B. A. Frenz, J. R. Ebner, and R. A. Walton, lnorg. Chem., 15, 1630 (1976). (4) M. H. Chisholm. F. A. Cotton, M. Extine. M. Miiiar, and B. R. Stults, Inorg. Chem., 15, 2244 (1976); J. Am. Chem. SOC:, 98, 4486 (1976); M. H. Chisholm, F. A. Cotton, B. A. Frenz, W. W. Reichert, L. W. Shire, and B. R. Stults, ibid., 98, 4469 (1976); M. H. Chisholm, F. A. Cotton, M. Extine, and B. R. Stuits, lnorg. Chem., 15, 2252 (1976); 16, 320, 603 (1977). (5) F. A. Cotton, J. D. Jamerson, and B. R. Stults, J. Am. Chem. Soc., 98, 1774 ( 19 76).
/
100.1 3
/
June 21, 1978
(6) B. H. Byers and T. L. Brown, J. Am. Chem. SOC.,97, 947 (1975). (7) E. L. Muetterties, B. A. Sosinsky. and K. I.Zamaraev, J. Am. Chem. SOC., 97, 5299 (1975). (8) The covalent radius of iron was estimated in a manner analogous to that used for chromium in ref 9. (9) R. D. Adams, D. E. Collins, and F. A. Cotton, J. Am. Chem. Soc.. 96, 749 (1974). (10) 0. S. Mills and J. Robinson, Acta Crystallogr., 16, 757 (1963). (1 1) (a) G. H. Stout and L. H. Jensen, "X-Ray Structure Determination, a Practical Guide", Collier-MacMillan, London, 1968. p 419; (b) J. A. Kaduk. A. T. Poulos, and J. A. Ibers, J. Organomet. Chem., 127, 245 (1977). (12) A. Nakamura, PJ. Kim, and N. Hagihara, J. Organomet. Chem., 3, 7 (1965); S. Otsuka, A. Nakamura, and K. Tani, J. Chem. SOC.A, 154 (1971); N. Yasuda. Y. Kai, N. Yasuoka, and N. Kasai, J. Chem. SOC.,Chem. Commun., 157 (1972). (13) R. Ben-Shoshan and R. Pettit, Chem. Commun., 247 (1968). (14) P. Mbener and E. Weiss, J. Organomet. Chem., 129, 105 (1977); G. Dettlaf and E. Weiss, ibid., 128, 213 (1976). (15) R. P. Dodge and V. Schomaker, J. Organomet. Chem., 3 , 2 7 4 (1965). (16) C. Casey and C. Cyr, J. Am. Chem. SOC.,95,2248 (1973). (17) H. W. Sternbera, R. A. Friedel. R. Markbv, and I. Wender. J. Am. Chem. SOC.. 78, 3621 (195;). S. Otsuka, K. Mori, T. Sumino, and F. Imaizumi. Eur. Polym. J., 3, 73 (1967). D. T. Cromer and J. T. Waber, "International Tables for X-Ray Crystailography", Vol. IV, Kynoch Press, Birmingham, England, 1974, Table 2. D. T. Cromerand D. Liberman, J. Chem. Phys., 53, 1891 (1970). A set of computer programs from Picker Nuclear, Inc., was utilized for automatic operations of the diffractometer. The general plane Fourier mapping program, JIMDAP, modified by J. lbers and F. Ross from the original version FORDAP by H. Zalkin and D. H. Templeton, was used to calculate Fourier maps. Least-squares programs were a local version of ORFLS by W. R. Busing, K. 0. Martin, and H. A. Levy and ORXFLSI based on ORFLS with modifications by R. D. Ellison, W. C. Hamilton, J. A. Ibers. C. K. Johnson and W. E. Thiessen. Standard deviations were calculated using DRFFEJ modified by G.M. Brown, C. K. Johnson and W. E. Thiessen from ORFFE by W. R. Busing, K. 0. Martin, and H. A. Levy. ORTEP by C. K. Johnson was used to plot the molecular structures and theoretical hydrogen atom positions were calculated from known atomic positions and hybridizations using HYGEN by F. K. Ross. Angles between planes and deviation from planes were calculated by MEAN PLANE by M. E. Pippy and F. R. Ahmed. C. K. Johnson, Acta Crystallogr., Sect. A, 25, 187 (1969). Supplementary material.
Photodissociation of Molecular Beams. Cleavage of Metal-Metal Bonds in Rhenium and Manganese Decacarbonyl Andrew Freedman and Richard Bersohn* Contribution from the Department of Chemistry, Columbia Uniaersity. New York, New York 10027. Received December 16, 1977
Abstract: Photodissociation of rhenium decacarbonyl has been studied in a molecular beam. The energy distribution of the photofragments obtained with incident laser light at 300 nm proves that photodissociation involves only metal-metal bond cleavage. Furthermore, two-thirds (-30 kcal/mol) of the available energy is found as internal energy (probably vibrational) of the photofragments, with the balance being translational energy. Angular distributions of the photofragments as a function of polarization of incident light were measured for both rhenium and manganese decacarbonyl. The anisotropy parameter, @, obtained from the distributions has the same value, 1.9 f 0.3, in both compounds; this value proves that the first strong ultraviolet absorption bands involve parallel transitions and puts upper limits of several picoseconds on the lifetimes of the excited electronic states.
Photodissociation of a molecular beam offers unique advantages in unraveling the complexities of excited electronic states. These experiments, performed in the gas phase under collision-free conditions, detail the polarization of the involved transition, set limits on the lifetime of the dissociating state, identify the photofragments, and produce information as to energy partitioning in these photofragments without the ambiguities produced by solvent This technique has been applied to two dimetal decacarbonyls ( ( C 0 ) s M M(CO)s, M = Re, Mn) whose photochemistry offers unusual synthetic pos~ibilities.~-lBoth molecules have their first strong 0002-7863/78/1500-4116$01.00/0
absorption band in the near ultraviolet, the rhenium compound a t -33 000 cm-I and the manganese compound a t -29 000 cm-'; these transitions are believed to involve excitation and cleavage of the M-M bond.'0%12
Experimental Section The apparatus and experimental procedures have been extensively described e l ~ e w h e r e . ~I % 6 n brief, a molecular beam is intersected at right angles by a chopped or pulsed beam of polarized light. A small fraction of molecules are photodissociated; the resulting photofragments are ionized by an electron bombardment ionizer, mass selected
0 1978 American Chemical Society
41 17
/ Rhenium and Manganese Decacarbonyl
Freedman, Bersohn
TlME
200
,lOC
-21 0
++ t
*
i 5 1
+*
t++++
I
+++j
400 I
I
I
+
+
4
+t*t
+++
FLIGHT Im8crasrrandil
I
I
4
+
++
OF
300
1
+ ++
+ i + + + I
I
1
I
I
I
by a quadrupole mass filter, and detected by an electron multiplier, all of which are housed in a differentially pumped vacuum chamber along the third orthogonal axis. In the angular distribution mode, the light is produced by a kilowatt high-pressure mercury-xenon vapor lamp and chopped at 250 Hz. All wavelengths below 290 nm are removed by an inorganic filter solution13 (to avoid exciting higher energy absorption bands) and the resulting beam is polarized by a plastic ultraviolet polarizer. The photofragment signal is detected in the analog mode and processed by a lock-in amplifier. When a time-offlight (TOF) spectrum is measured, pulsed ultraviolet polarized light is produced by a frequency doubled flashlamp-pumped dye laser (Chromatix CMX-4). The signal, in the digital mode, is recorded in a multichannel scaler as a function of time after the laser pulse. The energy distribution of the photofragments is derived from the TOF spectrum, the distance between the beam-light interaction zone and detector, and the masses of the photofragments. Relevant experimental parameters are given in Table 1. The chemicals were bought commercially and used without further p u r i f i c a t i ~ n . ' ~
Results A. Angular Distributions. Photofragment intensity as a function of angle of the polarization of light has been shown to be fit by the following function:
[
+ /3
G
'>1
-cos28 - 2 where N is a normalizing constant; 8 is the center of mass scattering angle relative to the polarization axis; and /3 is an anisotropy parameter which can range from +2 for a parallel transition to -1 for a perpendicular transition. The data shown in Figure 1, when corrected for instrumental distortion and incomplete polarization of the incident light, yields a /3 value of 1.9 f 0.3 for both compounds. This not only confirms the assignment of both transitions as being purely parallel,I5 but also places an upper limit on the lifetime of the excited electronic state(s). If the lifetime were as long as a characteristic molecular rotation (several picoseconds for this molecule), the large anisotropy would not have been preserved. B. Energy Distributions. The translational energy distribution of the Re(CO), photofragments is shown in Figure 2. Although the detected ion was Re+, the energy was calculated assuming that the parent photofragment was Re(CO),; this assignment will be discussed later. On the average, only 15 kcal/mol, Le., 33% of the available energy, is found in the translational mode. A TOF spectrum of the Mn(C0)S frag[(e) = N 1
Figure 2. Time of flight (TOF) spectrum (top) and translational energy distribution (bottom) of Re(C0)5 fragments from photolysis at 300 nm. The flight path was 11.2 cm. A detector response time of 47 1 s has already been subtracted from the data. Table I. Experimental Parameters"
Source pressure Beam temp M-M bond strengthd Laser wavelength Available energye
Re-Re
Mn-Mn
0.096 390 51.1 300 45.7
0.80C 365 22.1 (320) (73.1)
Pressure is in Torr, temperatures in K, energies in kcal/mol, wavelength in nm. A. K. Baev, V. V. Dem'yanchuk, G. Mirzoev, G. I. Novikov, and N. E. Kolobova, Russ. J . Phys. Chem. (Engl. Transl.), 45, 777 (1971). F. A. Cotton and R . R. Monchamp, J . Chem. Soc., 533 (1960). Reference 16; recent photoelectron spectra results (M. B. Hall, b. Am. Chem. SOC.,97,2057 (1975)) lower the first ionization potential of Re*(CO)l,-, by 4.6 kcal/mol. This might raise the Re-Re bond strength by that amount. e E,,,,I = hw Do 2RT.
+
-
ment could not be obtained because of poor signal to noise ratios. Among other problems, the product of the laser power and the molar extinction coefficient of Mn2(CO)lo, although optimized at 320 nm, was a factor of 10 lower than the corresponding product for Re2(CO)lo a t its optimum wavelength of 300 nm.
Discussion A. Metal-Metal Cleavage in Rez(CO)lo. The T O F spectrum of Re+ proves that the dissociation process involves only the cleavage of a metal-metal bond and not a metal-carbon bond. If one or more metal-carbon bonds were broken, conservation of momentum and energy would require the fragments to have far more than the available energy (97 kcal/mol minus bond energies). Moreover, the 95 kcal/mol photon has insufficient energy to break both the Re-Re bond (51.1 kcal/mol)I6 and a Re-C bond (39.1 kcal/mol)16 and still generate an average kinetic energy of 15 kcal/mol. It should be emphasized that M n z ( C 0 ) 1 0 might present a different picture. The available energy is much higher (-73 kcal/mol) and the metal-carbonyl bond much weaker (-20 kcal/mol).I6 B. Energy Partitioning. A preponderance of available energy, 32 kcal/mol on the average, appears as internal energy of the two Re(C0)s fragments. This internal energy could, in principle, be partitioned among the 27 vibrational and 3 ro-
41 18
Journal of the American Chemical Society / 100.1 3 / June 21, 1978
tational degrees of freedom, and even the electronic energy. In fact, it will be argued, the energy appears mainly in three particular vibrational modes. First, simple pictures of the bonding in the Re(C0)5 radical do not predict any very low lying excited electronic states. Second, the electronic transition in the parent molecule involves the promotion of a u bonding electron to an antibonding u* state;l2.l5this is accompanied by a switching on of a repulsive potential between the Re atoms which pushes them apart but preserves the fourfold symmeThus no torques are produced which would generate rotational energy; the internal energy will be entirely vibrational energy (or almost so). Moreover, the preservation of symmetry requires that only the six symmetric al vibrations be excited. Of these six normal modes, two correspond to very high frequency (-2000 cm-l) stretching modes of the C O ligands. These frequencies differ only slightly in different metal carbonyls and consequently they are expected to be similar in Re2(CO)lo and in Re(C0)5.18 If so, the dissociation is not accompanied by any strong change in potential energy between the carbon and oxygen atoms which could excite the C - 0 stretch. According to our simple picture of the electronic transition, the bonding of the rhenium atom to the four equatorial C O ligands is not changed upon dissociation; however, the unpaired electron, formerly used for a Re-Re u bond, is now in a u antibonding state between a rhenium and an axial carbon atom. Thus a sharp change in the shape of the potential between the Re and C,, atoms is expected which will excite this stretching mode. The remaining three modes are a symmetric stretch in which all four equatorial Re-C bonds stretch in phase; a symmetric equatorial carbonyl rock; and a symmetric bend in which all four equatorial COSflap up and down in phase through their original plane. The sudden motion of the Re atom, on geometric grounds, causes a larger change in the C,,-Re-C,, and O-Ceq-Re bond angles than in the Re-C,, bond distance, thus exciting the symmetric bend and rock. Quantum mechanically, one might describe the two excitations in the following manner. The axial stretch is a consequence of Franck-Condon factors which correlate the change in vibrational state with the change in vibrational potential. The symmetric equatorial bend and rock excitation is caused by the Re-Re repulsive potential in the excited state. The
relative magnitudes of the two excitations can be crudely estimated. The second, or dynamic, part can be roughly calculated by the impulsive spectator model19 in which the momentum of recoil is supposed to be received initially by the Re-CO,, group leaving the four equatorial COS as initially unaccelerated bystanders. Subsequently, the Re-CO,, group is slowed down by its interaction with the four equatorial COS with resulting vibrational excitation. The fraction of total energy predicted as being vibrational in this approximation is just 4 M c o / ( M ~ , SMco) or 0.34. This would leave -I/3 of the energy in the Re-CO,, stretch modes and l/3 in translation. The discussion above is pertinent to the initially generated vibrational distribution and not to the succeeding phenomenon of intramolecular vibrational relaxation.
+
Acknowledgments. W e would like to thank Professor W. G. Klemperer for fruitful discussions concerning bonding in dimetal decacarbonyls and the National Science Foundation for Grant C H E 76-17581 which supported this work. W e would also acknowledge Dr. Masahiro Kawasaki for his help in collecting some of the data. References and Notes M. Dzvonik, S. Yang, and R . Bersohn, J. Chem. Phys., 61,4408 (1974). S. Yang and R. Bersohn. J. Chem. Phys., 61, 4400 (1974). S. J. Riley and K. Wilson, Discuss. Faraday SOC.,53, 132 (1972). , , G. F. Busch and K. R. Wilson, J. Chem. Phys., 56, 3626 (1972). (5) M. J. Dzvonik and S. C. Yang, Rev. Sci. Insburn., 45, 750 (1974). (6) M. Kawasaki, S. J. Lee, and R. Bersohn, J. Chem. Phys., 66, 2647 (1977). (7) H. B. Abrahamson and M. S. Wrighton, J. Am. Chem. Soc., 99, 5510 119771 \ . - . . I .
(8) B. H. Byers and T. L. Brown, J. Am. Chem. SOC.,99, 2527 (1977). (9) D. M. Alien, A. Cox, T. J. Kemp, Q.Sultana, and R . B. Pitts, J. Chem. Soc., Dalton Trans., 1189 (1976). (10) M. S. Wrighton and D. S.Ginley, J. Am. Chem. Soc., 97, 2065 (1975). (11) J. L. Hughey, C. R . Bock, and T. J. Meyer, J. Am. Chem. Soc., 97, 4440 ( 1975). (12) R. A. Levenson and H. B. Gray, J. Am. Chem. SOC., 97,6042 (1975). (13) Filter solution: 0.025 M CuS04 (1 % H2S04);3-cm cell. (14) Alfa Products, Ventron Corp., Danvers, Mass. (15) R. A. Levenson, H. B. Gray, and G. P. Caesar, J. Am. Chem. Soc., 92,3653 (1970). (16) H. J. Svec and G. A. Junk, J. Am. Chem. Soc., 89,2836 (1967). (17) L. F. Dahl, E. Isishi, and R . E. Rundle, J. Chem. Phys., 26, 1750 (1957). (18) H. Huber, E. P. Kundig, G. A. Ozin, and A. J. Poe, J. Am. Chern. SOC., 97, 308 (1975). (19) R. D. Levine and R. B. Bernstein, "Molecular Reaction Dynamics", Oxford University Press, London, 1974.
